WO2010079289A1 - Intramedullary anchoring stem for an orthopaedic implant head - Google Patents

Abstract

An intramedullary anchoring stem (7) for an orthopaedic implant head. The stem is in one piece, is made of biocompatible material and is movable in relation to the head, with which it has connecting means (16), and it has a truncated or substantially truncated peg shape (11) at least partially recessed (21) longitudinally, and parallel or substantially parallel to the dorso-palmar plane (22).

Description

 INTRA MEDULLAIRE ANCHORING ROD FOR ORTHOPEDIC IMPLANT HEAD

The present invention relates to an intramedullary anchoring rod for an orthopedic implant head used in digital arthroplasty.

It also relates to a method of setting up such a rod.

It finds a particularly important, though not exclusive, application in the medical field of prostheses of the

Phalangian Proximal of the hand (known in the field by its initials IPP).

But it is also applicable more generally to the joints between other types of so-called proximal phalanx and an adjacent distal phalanx.

By so-called proximal phalanx is meant a phalanx or a metacarpal or metatarsal bone element located on the side of the body or limb (hand or foot) concerned, and by phalanx distal a phalanx located towards the outside of this limb, according to the conventions of orientation adopted conventionally in anatomy, and which will also be used hereinafter. The invention can thus be used for inter-phalangeal digital joint prostheses (IP inch) or for distal interphalangeal (IPD) prostheses, for the hand or for the foot. It can also be advantageously used for metastatic digital joint prostheses. carpophalangeal (called MCP), trapezo-metacarpal (so-called MTP), here again for the hand or for the foot, and this by simple geometric adaptation to the bone sites concerned. Interphalangeal articulation prostheses are already known comprising first and second elements each comprising respectively an implantation rod in the bone and a complementary inter-phalangeal articulation head. For example, it is known (EP 1 339 362) an implant comprising a first element provided with a head having a convex surface of bicondylar articulation and a second element provided with a complementary head. The elements of such prostheses consist of a piece, the rod having a solid pile-form integral with the head and arranged to be introduced inside the medullary cavity of a phalanx. The problem solved by this type of prosthesis is that of the wear of the articular surfaces, the stem itself being considered as secondary.

Also known (EP 1 870 061), prostheses comprising tapered anchoring rods elongate about an axis.

Here again the problem of wear of the prostheses is strictly solved thanks to an adaptation of the articulation surfaces between them.

Also known (US 2003/0040805) prostheses in three parts namely, a head, a spacer and a hollow rod whose end is duckbill shaped two flexible strips deformable between a position of introduction into the bone and an anchoring position where the spacer blocks the prosthesis. Such a system is complicated to implement.

The present invention aims to provide an orthopedic implant and a method that better than those previously known to the requirements of the practice, in particular because it generates very low wear despite numerous articulation movements, and in that great simplicity and ease of assembly by adapting to the constraints related to the subsequent operations of arthroplastic joints. To do this, it starts from a different approach to that of the prior art.

Rather than take for granted the need for a solid and rigidly rigid implantation rod, to ensure the anchoring of a joint whose wear is then limited by the adoption of a particular shape of the heads, the present The invention first of all accepted the questioning of the very design of the rod secured to the head.

It has therefore first made removable from the head, which allows including the use of two different materials and replacement of the head without having to remove the rod.

The invention then provided a rod in one piece including clean to be implanted, without screwing into the intramedullary canal of the bone. The invention finally and in combination with this dissociation and rigid connection in one piece, made the rod itself more flexible in the dorso-palmar plane by providing a rod with apparent elasticity and rigidity close to the bone in which it is implanted.

All of these elements taken in combination will cause a more natural behavior of the heads against them, with the unexpected consequence of considerably limiting their wear while presenting the advantages of the known one-pieces in terms of strength, that is to say without the fragility and complexity of the multi-piece implants of the prior art. In other words, the invention makes it possible to propose a range of intra-medullary anchoring rods of great strength but whose deformation under stress is equivalent to that of the bone in the sagittal (anteroposterior) plane, plane following which the physiological constraints are mainly transmitted during the loading of the joint (pinching, gripping, ...).

The so-called "stress-shielding" stress phenomena associated with the differences in rigidity between the bone and the prosthetic parts are therefore minimized, favoring the osteointegration of the intramedullary rod.

For this purpose the present invention provides an intra-medullary anchoring rod for an orthopedic implant head, removable relative to the head with which it comprises connection means, said rod comprising a body at least partially hollowed longitudinally, parallel to or substantially parallel to the dorso-palmar plane, characterized in that the rod is formed in one piece by said body, and in that it has a frustoconical pile shape or substantially frustoconical biologically compatible material.

Thanks to this modularity, a shape of the lightened rod in the dorso-palmar plane, the latter being manufactured in a biologically compatible material determined, the rod is thus more flexible in this plane and better adaptable to the knuckles concerned, which limits the wear of the prosthesis by allowing a better and less traumatic grip of the bone on the stem.

Particularly advantageously the longitudinally recessed portion is arranged to give said rod of determined shape and material elasticity and / or rigidity corresponding to determined values close to those of a bone, and in particular of a finger bone or of foot.

The determined values may be chosen by those skilled in the art according to the particular characteristics of the patient's bone to be treated, as described below with reference to the so-called "effective" or apparent Young's modulus.

In advantageous embodiments, one and / or the other of the following provisions is also used: the stem has a non-zero radius of curvature arranged to fit inside the medullary cavity of a phalanx; it has an end stop anti-depression;

- The shape of the rod at least partly hollowed out and its material give it an effective Young's modulus E _eff <30 Gpa;

By effective or apparent Young's modulus is meant a global Young's modulus of the stem whose behavior is identical to that of a bone element.

In practice, and to calculate the effective Young's modulus as envisaged in the present invention, the bone is assimilated to a cortical tube of Young's modulus of the order of 20 Gpa, the spongy part therefore not participating. no or little resistance.

This tubular bone is then transformed into a homogeneous material, that is to say into a solid base which then has said effective Young's modulus.

For example, for a young bone (≤ 35 years), the thickness of the tube is of the order of 1.5 mm, and the effective Young's modulus of the equivalent solid rod is then of the order of 16 Gpa.

For an older person, the thickness of the cortex is closer to 1 mm and the equivalent module is then of the order of 13 Gpa.

The effective or apparent Young's modulus of the stem is thus the combination between the intrinsic modulus and a geometrical factor; the effective Young's modulus is E _eff <20 Gpa, for example 16 Gpa;

the effective Young's modulus is E _Θff <15 Gpa, for example 12 or 13 GPa; the stem has a cross section in H;

it has a cross section in U or V;

- it has a duckbill slot in the dorso-palmar plane; - It has a recessed half-cylinder shape on the palmar side or half-tube; the connection means are formed by a conical fitting of a pin secured to the head in a recess in the corresponding rod. The recess is of complementary shape to that of the pawn;

- the stem is dissimetric with respect to a median plane passing through the axis of the obvious, parallel to the dorso - palmar plane; - The conical fitting is a Morse cone, ie with a taper angle of the order of 5%: but this taper may also be lower, for example of the order of 3% or 2.5 %

- The connecting means between the rod and the corresponding head further comprises a recess or a groove formed in one of the parts adapted to receive a complementary shaped plate integral with the other part;

the stem comprises at least partly an osteo-integrative coating increasing the anchoring; the rod is made of microporous titanium and / or is coated with hydroxyapatite;

- The rod is perforated by at least one transversely traversing transversely or substantially transversely said rod.

The invention also proposes an implant comprising an intra-phalangeal articulation head comprising a rod as described above.

Advantageously, the material of the rod is different from the material of the hinge head.

More precisely, two apparent stiffnesses or effective Young's moduli of different stems can be provided according to the patients, ie a rigid stem for the young patients (ie less than of the order of 30 GPa) and a more flexible stem for the patients. aged (ie less than of the order of 15 GPa), the head being in turn of lower Young's modulus polyethylene, for example <5 Gpa, for example 2 Gpa. Finally, the invention proposes a set of implants for which the connection means are identical between firstly, several heads of different sizes and secondly, identical or different rods of first elements and / or second elements.

Such an arrangement allows a great modularity.

The invention also proposes a method of placing a stem of an orthopedic implant for digital arthroplasty of the type described above. It will be better understood on reading the following description of embodiments given below by way of non-limiting examples.

The description refers to the accompanying drawings in which:

Figure 1 is a schematic top view of a hand skeleton in which implant rods according to the invention have been put in place.

Figure 2 shows in perspective an implant rod according to a first embodiment of the invention.

Figures 3 and 4 are perspective views of examples of heads, bi-convex and bi-concave adapted to fit the rod of Figure 2. Figure 4 is a schematic view from above (or dorsal) showing an implant according to the invention, elongate joint.

FIGS. 5A and 5B are cross-sectional diagrams of the hinge heads, respectively in the bent finger position, and the right finger, illustrating the flexibility of the rods for minimizing the wear of the articulation surfaces.

Figure 6 is a side sectional view of an implant member comprising a stem and a head according to an embodiment of the invention.

Figures 7 to 11 show in rear perspective, five embodiments of implant rod according to the invention.

Figure 1 is a schematic top view of a hand 1 skeleton, comprising orthopedic implant rods 2 according to the invention. The implant comprises a first element 3 for proximal phalanx 4 and a second element 5 for distal phalanx 6.

Each element comprises respectively an implantation rod or anchoring in the bone 7, 8 and an articulating head 9, 10 inter-phalangeal.

The same reference numbers will subsequently be used to designate similar or identical elements. FIG. 2 shows the removable anchor rod 7 with respect to the head 9 of the first element 3 (see FIG. 3) made of a bio-compatible material, for example made of titanium.

It comprises an elongate portion 11 substantially around an axis 12, slightly curved, for example with a radius of curvature p of 100 mm for a rod for proximal element of large size and 90 mm for proximal element of medium or small size or still between 50 mm (large size), 40 mm (medium size) and 10 mm (small size) for distal elements.

The rod is provided with a portion 13 arranged to be introduced into the central hole of the spinal cord of the phalanx, of substantially frustoconical polygonal shape, for example hexagonal with an enlarged end portion 14 with a facet 15 allowing a good introduction and a lateral locking in indexing in the modular cavity of the phalanx. The rod 7 is removably mounted relative to the head and comprises means 16 for connection with said head.

These connection means comprise a conical orifice 17 into which a conical pin is fitted.

18 (see Figure 5A) secured to the head 9, for example a Morse taper or having a taper angle of 2 to 3 degrees.

Pion and recess are complementary shapes and cooperate friction with each other.

The rod further comprises an anti-recess plate 19 that cooperates with a complementary slot-shaped recess 20 (see FIG. 3), which makes it possible, when the stud 18 is pushed into the orifice 17, and blocking the plate 19 into the slot. obviously 20, a good fastening and good indexing of the head 9 with the rod 7.

According to the invention, the rod 7 further comprises at least one recessed portion 21 in the dorsopalar plane 22, which gives it the desired flexibility and which will be detailed hereinafter.

Figure 3 shows the head 9 of the first element 3.

It has a bi-convex condylar articulation surface 23 comprising a central valley 23 '.

Figure 4 shows the head 10 of the second element.

This has a bi-concave surface 24 arranged to cooperate with the bi-convex surface 21 of the head of the first element and provided with a central ridge 24 '. The surfaces 23 and 24 are, for example, arranged to be congruent at the level of the two condyles 25 and 26 and the corresponding concave surfaces 27 and

28 in the frontal plane of the joint and not congruent in the sagittal plane.

But all other types of head are perfectly conceivable.

FIG. 3 shows the contact surface obtained with a head of first element made of elastic material when longitudinal pressure is exerted.

The line of friction in the frontal plane is effectively transformed into a surface 29, which makes it possible to better distribute the friction forces, and thus to minimize end wear of the head.

By virtue of the combination of the material of this head, which may for example be a UHMWPE-type polymer, the respective radii of curvature mentioned above, and characteristics of the invention, a further improved damping and a reduced wear of the prosthesis in time.

It is noted in this regard that the contact zone 29 scans half of the condyle but remains at about the same place on the distal part. Advantageously, the most fragile surface is then selected on the condyle side, the flexibility of the rods in the dorso-palmar plane also allowing even better distributed wear as indicated above. FIGS. 5A and 5B show the evolution of the flexibility of the rods 7 and 8 at course of movement, flexed finger (Figure 5A) and right finger (Figure 5B).

The fact of standardizing and standardizing the dimensions of the pins 18 and orifices 17 also makes it possible to adapt indifferently a head of a certain dimension with a rod of different size which allows a great modularity.

Other means of mechanical assembly of the head on the rod are of course possible. FIG. 6 is a sectional view of a first element 30 comprising the head 9 and the rod 7, the counter

18 snapping into the hole 19.

The obviously 21 allows a good flexibility

(arrow 31) in the plane 32 perpendicular to the dorso-palmar plane 22.

Figures 7 to 11 show stems for distal or proximal element of different shapes and lengths according to different embodiments of

The invention. The rod 33 of FIG. 7 has on one side an anti-depression plate 34 of the type described with reference to FIG. 2, pierced with a hole for receiving the pin of the head.

The solid portion 35 is extended in the opposite direction to the plate of a portion 36 in a flat bevel provided with a through slot 37 over its entire length of thin, for example 2 mm in the dorsal plane palmar, which gives it a normal form of duckbill. FIG. 8 shows another embodiment of the rod 38 representing a part 39 suitable for being embedded in the bone, substantially pyramidal or frustoconical H-shaped cross-section, thus releasing on either side of the dorsopalar plane 40 substantially parallelepipedal recesses, from the tip 41 to the plate 42 anti-sinking.

Figure 9 shows another rod 43 comprising a portion 44 of frustoconical shape provided on the lower surface of a chute 45 leading to the rod shaped recessed about 2/3 to 9/10 ^th of its length.

FIG. 10 shows in turn a half-rod 46 in the form of a substantially cylindrical tube 47 comprising an end portion 48 provided with the plate 49, pierced with a recess 50 to cooperate with the pin of the head as described previously, the recess 50 being extended by a bore 51 over the entire length of the tube 47.

Fig. 11 shows a folded sheet metal rod 52 having an end portion 53 of U-shaped cross-section.

In the embodiments of the invention more particularly described here, the rods are rough. This roughness is for example obtained by sanding or corunduming, or by transverse grooves (not shown), to increase the adhesion in the bone.

In order to further improve the anchoring, an osteo-integrative coating is advantageously provided by example of the Hydroxyapatite type (HAP initials) or formed of microporous titanium by plasma deposition.

We will now describe the establishment of a prosthesis according to the invention during an implantation operation.

After opening the finger and section of the damaged bone parts so as to place the prosthesis, in a manner known per se, (first bone resection step) the housing of the rods (introduction of rasps) are prepared, then introduced into the medullary canal the fixation rod.

We then check the space available for the placement of the heads and then fits the condylar proximal head 9 which is fixed precisely and blocked through the Morse taper on said rod.

A test is then performed with a phantom of the other distal head.

Then, after the choice of the thickness according to the pre-resection of the bone, is placed on the other side, the rod 8 in the distal phalanx and the joint is then finalized by setting up the head 10.

The fact that the head 10 can be chosen with several different thicknesses for its base allows adjustment during the operation.

All of these operations are carried out in a manner known per se by means of dedicated ancillaries.

As is obvious and as also follows from the above, the present invention is not limited to the embodiments more particularly described. She embraces contrary to all variants and in particular those where the anchor rods are non-metallic, for example bio-compatible polyethylene, or are in a metal other than titanium, those where the prosthesis is an IP prosthesis thumb or big toe, or a IPD, or meta-carpophalangeal prosthesis which involves different dimensions and a different articular surface design so as to allow adequate lateral mobility.

Claims

1. Rod (2, 7, 8, 33, 38, 43, 46, 52) intramedullary anchoring for orthopedic implant head, removable from the head (9, 10) with which it comprises means ( 16), said rod comprising a body at least partly hollowed out (21) longitudinally, parallel to or substantially parallel to the dorso-palmar plane (22), characterized in that the rod is formed in one piece by said body, and in that it has a frustoconical (13) or substantially frustoconical shape of a biologically compatible material. 2. Rod according to claim 1, characterized in that the longitudinally recessed portion is arranged to give said rod of determined shape and material elasticity and / or rigidity corresponding to determined values close to those of a bone. 3. Rod according to any one of the preceding claims, characterized in that said rod being substantially elongated about an axis, it has a non-zero radius of curvature arranged to fit inside the medullary cavity of a phalanx. 4. Rod according to any one of the preceding claims, characterized in that it has an end stop anti-depression. 5. Rod according to any one of the preceding claims, characterized in that it has a shape at least partly hollowed out and consists of a material giving it an effective Young's modulus E _eff <30 Gpa. 6. Rod according to claim 5, characterized in that it has an effective Young's modulus E _eff <15 Gpa. 7. Rod (38) according to any one of the preceding claims, characterized in that it has a cross section in H. 8. Rod (52) according to any one of claims 1 to 6, characterized in that it has a cross section in U or V. 9. Rod (33) according to any one of claims 1 to 6, characterized in that it has a slot (37) duckbill in the dorso-palmar plane. 10. Rod (43, 46) according to any one of claims 1 to 6, characterized in that it has a recessed half-cylinder shape on the palmar side or half-tube. 11. Rod according to any one of the preceding claims, characterized in that the means (16) of connection are formed by a conical fitting of a pin (18) integral with the head in a recess (17) formed in the rod corresponding. 12. Rod according to claim 11, characterized in that it is concealed with respect to a median plane passing through the axis of the recess, parallel to the dorso-palmar plane. 13. Rod according to any one of claims 11 and 12, characterized in that the conical fitting is a Morse taper. 14. Rod according to any one of the preceding claims, characterized in that the means (16) for connection between the rod and the corresponding head further comprises a recess (20) or a groove formed in one of the parts specific to receive a plate (19) of complementary shape integral with the other part. 15. Rod according to any one of the preceding claims, characterized in that it comprises at least partly an osteointegrating coating increasing the anchoring. 16. Rod according to any one of the preceding claims, characterized in that it is made of microporous titanium and / or is coated with hydroxyapatite. 17. Rod according to any one of the preceding claims, characterized in that it is perforated by at least one transversely transverse or substantially transversely transverse said rod. 18. Implant comprising an inter-phalangeal articulation head, characterized in that it comprises a rod according to any one of the preceding claims. 19. Implant according to claim 18, characterized in that the head is made of a material different from the rod. 20. Set of implants according to any one of claims 18 and 19, characterized in that the implants comprise identical connection means between on the one hand, several heads of different sizes and on the other hand, identical or different rods.